1. Field of the Invention
The present invention relates to a lens barrel and particularly to a lens barrel provided with a focus ring for focusing operation and a mechanism for driving the focus lens by a motor according to a rotation position of the focus ring by electrically detecting the rotation position of the focus ring.
2. Description of the Related Art
A consumer video camera in general has an automatic focus function so that focusing can be controlled by switching between automatic focus and manual focus. Thus, a lens for focus control (focus lens) is driven by a motor, and a manual focus is carried out by electrically detecting the rotation position of a focus ring provided rotatably at a lens barrel by a position sensor, for example, and by moving the focus lens by the motor according to the detected rotation position. And the focus ring and the focus lens are so constructed that they are not mechanically interlocked with each other (See Japanese Patent Application Laid-Open No. 2005-121745, for example).
And in a lens barrel used in this kind of video camera, a position sensor of a relative position detection type such as an encoder is often used, not a position sensor of an absolute position detection type such as a potentiometer as the position sensor for detecting the rotation position of the focus ring. The focus ring is also different from the lens barrel generally used for broadcasting or professional use but is capable of endless rotation without its rotation range restricted by a close-up end and an infinite end. This has the following advantages. For example, when a target subject is out of focus in executing focus control by automatic focus, focus control by the focus ring might be desired by switching from the automatic focus to the manual focus. In this case, when the automatic focus is switched to the manual focus, the focus lens is preferably moved by a displacement amount according to an operation amount of the focus ring (rotation change amount) after being switched to the manual focus based on the focus position at that time. That is, if the focus position set by the automatic focus is effectively taken over to change the focus position by the manual focus, the manual focus can be used as a focus correcting device or the like for the automatic focus. Suppose that a position sensor of the absolute position detection type is used for detection of the rotation position of the focus ring and a limit (end) is posed on the rotation range of the focus ring, such a nonconformity is generated that the focus lens can not be moved to the close-up end or the infinite end even if the focus ring is rotated to the position at the end. That is, an actual focus position when the automatic focus is switched to the manual focus is deviated from the focus position which should be set for the rotation position of the focus ring at that time. And if the focus lens is moved by the displacement amount according to the operation amount of the focus ring after being switched to the manual focus as above, the deviation is kept in the subsequent operation of the focus ring. Therefore, a phenomenon occurs that even if the focus ring is operated to the position at the end of the rotation range, the focus lens is not moved to the close-up end or the infinite end. If a position sensor of the relative position detection type is used for detection of the rotation position of the focus ring and a limitation (end) is not posed on the rotation range of the focus ring, the above nonconformity can be avoided.
The infinite focus ring having its rotation position detected by a position sensor of the relative position detection type and capable of endless rotation shall be called as a relative position instruction type focus ring (which moves the position of the focus lens in a relative position relation with respect to the rotation position of the focus ring), instructing a relative position of the focus lens, and the focus operation by that shall be called as a relative position instruction type focus operation. On the other hand, the focus ring having its rotation position detected by the absolute position detection type position sensor and a limitation posed on the rotation range (having a regulation end) shall be called as an absolute position instruction type focus ring (which moves the position of the focus lens in an absolute position relation with respect to the rotation position of the focus ring), instructing the absolute position of the focus lens, and the focus operation by that as an absolute position instruction type focus operation.